It is shown that an electric potential exists between the surface and the bulk of ionic crystals. The potential is expressed in terms of the energies necessary for formation of lattice defects. The distribution of the potential and of lattice defects near the surface is calculated. The difference between the concentration of lattice defects near the surface and that in the bulk leads to a ``surface conduction.'' The magnitude of the potential, the thickness of the space-charge layer near the surface and the ``surface conduction'' are calculated for NaCl crystals using numerical values, calculated by Mott and Littleton [N. F. Mott and M. J. Littleton, Trans. Faraday Soc. 34, 485 (1938)] for the energies of formation of lattice defects. The effect of charged impurities in the crystals on the surface potential is discussed. The importance of the surface potential on diffusion, photoelectric effect, and photographic effect is mentioned.
Direct current electric fields applied to polycrystalline barium titanate cause slow changes with time of current, potential distribution, and coloration. At temperatures between 250° to 300°C and with several thousa.nd volts/em applied, the current increases initially, then decreases, and then increases again. An analysIs of the potential distribution indicates injection of a high conductivity region from the cathode whereby the fi~l? at th~ cathode is decreased and the field at the anode is increased, followed by injection of a low conductivity regIOn from the anode which travels to the cathode, causing a high field at the cathode and renewed injection of high conductivity from the cathode. Drift mobilities for the low conductivity region have been estimated.
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